1. Technical Field
The present invention relates to a method and apparatus for constructing a SPECT (single photon emission computed tomography) image that is a three-dimensional distribution image of a radio isotope (RI) which is injected into a biological body under medical examination, by detecting .gamma. (gamma) rays emitted from the injected radio isotope (RI).
2. Background Art
Conventionally, there are available a computer tomography (CT) apparatus and a single photon emission computed tomography (SPECT) apparatus as one which reconstructs image by detecting radiation.
In the CT apparatus, there are provided an X-ray source and a group of detecting elements arranged in a geometric position where the X-ray source lies in a focus point of an X-ray fan beam. The X-ray source and the detecting elements are rotated around the longitudinal axis of the biological body so that an X-ray permeated through the biological body is detected by the group of the detecting elements so as to acquire projected data. Then, X-ray absorption distribution is image-reconstructed in accordance with the projected data thus acquired.
In SPECT apparatus, in order to detect .gamma. rays emitted from a radio isotope (RI) injected into a patient, there are provided a fan-beam collimator constructing a hypothetical focus, a scintillation detector which converts selectively transmitted .gamma. rays into light rays so as to be detected thereby. The fan-beam collimator and the scintillation detector constitutes a gamma camera. The distribution of radio isotope (RI) is image-reconstructed in accordance with the projected data acquired by the gamma camera, in a manner that the absorption of the .gamma. rays are neglected.
In either CT or SPECT apparatus, there is often utilized a filtered back projection technique. An example for the filtered back projection technique, there is a publication entitled "CONVOLUTION RECONSTRUCTION TECHNIQUES FOR DIVERGENT BEAMS" by G. T. Herman et al., Comput. Biol. Med. Vol. 6;1976, pages 259-271,". In this above publication, the X-ray fan beam is exposed to the patient from the X-ray source which rotates around the patient. The X-ray transmitted through the patient is detected by a detecting unit in which a plurality of the detecting elements are arranged in a linear or circular arc shape. Projection data thus detected are corrected by a filtering process. Thereafter, the filtered data are backprojected to entire effective field of view along an X-ray beam so as to reconstruct an X-ray CT image. In this case, the X-ray source lies in the focus, so that image information from all directions of 360.degree. are regarded equivalent in a relative manner.
However, in a case where the image is reconstructed by employing the filtered backprojection technique in the SPECT using the fan-beam collimator, a spatial resolution near the collimator is comparatively good while the spatial resolution deteriorates as a region is away from the collimator. In other words, an RI distribution located far away from the collimator does not sufficiently contribute to information on the projection data, thus resolution of the image thus constructed from the far-away region being very deteriorated.